The present invention relates generally to alignment detection apparatus, and more particularly to an apparatus for optically detecting the alignment of a substrate such as a reticle in reduction projection aligners for superfinely machining a semiconductor wafer or the like.
As an alignment detection apparatus to be used in reduction projection aligners including a mercury lamp acting as a continuous emission source, there is conventionally known an apparatus with an arrangement as disclosed in the Japanese Patent Provisional Publication No. 53-77009. This conventional alignment detection apparatus will be described hereinbelow with reference to FIG. 1. Although the alignment is effected with respect to three axes, i.e., the X- and Y-axes of the orthogonal directions and the .theta.-axis of the rotational direction, the description will be made only in terms of the alignment of the reticle in the X-axis direction. In the case of the requirement of the alignment accuracy, it is preferable that the exposure wavelength is coincident with the wavelength of the light to be used for the alignment.
In FIG. 1, illustrated at numeral 101 is an illumination optical system for flow-dividing light emitted from a mercury lamp acting as an exposure light source. The light from the illumination optical system 101 is reflected by a beam-splitter 103 toward an objective lens 104 side. After being passed through the objective lens 104, the light is reflected by a mirror 105 and then incident on a reticle 106. As illustrated in FIG. 2, the reticule 106 is arranged to be movable in a plane in the X- and Y-directions which are orthogonal to each other and further in the .theta.-direction which is the rotational direction so as to permit a fine alignment. At the portion on which the light reflected by the mirror 105 is incident there is provided a stripe-like alignment mark 107 (here, for the X-direction). The light incident on the reticle 106 is reflected by the alignment mark 107 so as to return the same path to advance to the mirror 105, objective lens 104 and the beam splitter 103. In this case, this light straightforwardly passes through the beam splitter 103 to reach an imaging lens 111. After being passed through the imaging lens 111, the light is separated by a beam splitter 112 into a straight-advancing light beam and a reflected light beam. The straight-advancing light beam passes through a target mark 113 and relay lens 114 so as to reach a vidicon 115 for the rough alignment. On the other hand, the reflected light beam from the beam splitter 112 is reflected by a vibration mirror 116 arranged to vibrate at a given frequency f. After being reflected thereby, the reflected light beam passes through a stripe-like target mark 117 and further passes through a condensing lens 118 so as to reach a light-receiving element (for example, photomultiplier) 119. An electric signal converted in the light-receiving element 119 is amplified by an amplifier 120 and the amplified signal varies as shown (1) to (9) in FIG. 3 with respect to the mirror vibration frequency in accordance with the moving amount (shifting amount) of the reticule 106 in the X-direction. Further, the amplified signal is inputted to a phase detector 121. To this phase detector 121 there is also supplied a signal with the reference frequency f from a mirror driver 122 which is a drive circuit for the vibration mirror 116. The phase detector 121 outputs a signal representative of the product of the inputted two signals. The output signal of the phase detector 121 is led to a low-pass filter 123 for averaging and is derived as a phase detection output.
FIG. 4 shows the relation between the moving amount of the reticle in the X-direction taken on the horizontal axis and the phase detection output taken on the vertical axis, where at the vicinity of the origin there is presented the most adequate alignment state.
Now, in order to meet the recent great requirement for high integration of simiconductor elements, it is required to shorten the wavelength of the exposure light, that is, it is required to use a KrF excimer laser (wavelength .lambda.=248 nm) or the like in place of the g-rays, i-rays. However, although the g- and i-rays can provide stable continuous waves, the KrF excimer laser offers an interrupted wave having the pulse width of 20 ns, the repeat frequency of 200 Hz and the output variation of .+-. about 5%, for example. Thus, there is a problem that difficulty is encountered to use this laser light for the conventional alignment detection apparatus arranged to use the continuous wave.